1. Field of the Invention
The present invention relates to controlling a vehicle provided with an automatic transmission.
2. Description of the Related Art
An automatic transmission of a vehicle is provided with a speed change controller, and the speed change controller controls a speed ratio and an oil pressure of the transmission. Some of the speed change controllers have a function to issue a command (torque reduction request) to an engine controller for reducing engine torque in order to suppress speed change shocks during upshifts and downshifts. The engine torque is temporarily reduced by a predetermined amount through use of an ignition timing retardation or a cutoff in fuel.
However, a period during which torque reduction is performed in the conventional example described above is only temporary. The torque reduction therefore cannot handle a situation in which torque reduction is requested continually. For example, if a state exists whereby the pressure supplied by a hydraulic pump does not reach a necessary pressure (due to extremely low temperatures or hydraulic system damage), it is necessary to perform torque reduction continuously to control the engine torque so that it does not exceed the amount of torque capable of being transmitted by the automatic transmission. However, there is such a risk that damage may be imparted to three-way catalysts in the engine, if ignition timing retardation or a cutoff in fuel is performed continuously.
Further, the amount of torque reduction is a predetermined amount that is set in advance and cannot respond to fluctuations in the amount of torque reduction required.
With a V-belt continuously variable transmission, an oil pressure is supplied to a pair of pulleys and torque is transmitted while a V-belt is gripped by the pulleys. The amount of torque that can be transmitted (torque capacity) is therefore always changing in accordance with changes in the operating state. If the torque capacity becomes smaller than the input torque, torque reduction corresponding to the amount of reduction in the torque capacity must be performed immediately in order to prevent V-belt slippage. However, it is difficult to perform torque reduction in accordance with the amount of reduction in the torque capacity with the conventional technique described above because the torque reduction amount is fixed. Further, after performing immediate torque reduction, it is necessary to perform torque reduction continuously for cases where the torque capacity drops rapidly due to damage of the hydraulic system or the like. The conventional technique described above cannot handle continuous torque reduction, however.